1. Field of the Invention
This invention relates to an anti-lock brake system, and more particularly, to a solenoid valve for an anti-lock brake system that can enhance a braking performance by variably controlling an amount of hydraulic oil to be supplied during a slip control.
2. Description of Related Art
Generally, a brake system comprises a wheel cylinder mounted on an automotive wheel to generate braking force using hydraulic pressure, a booster/master cylinder assembly for generating hydraulic pressure and applying the same to the wheel cylinder, and a modulator for controlling the hydraulic pressure to be supplied according to a running condition of a vehicle in accordance with signals transmitted from an electronic control unit.
The modulator is equipped with normal-open and normal-close solenoid valves for controlling the supply of braking pressure. The normal-open solenoid valve is designed to, in a normal brake mode, maintain an opened state to supply hydraulic pressure to the master cylinder without pressure reduction, and in a slip control mode, control an amount of the hydraulic oil to control the braking pressure.
The normal-open solenoid valve controls an amount of the hydraulic oil by a simple On/Off operation of a plunger. Accordingly, the flow rate of the hydraulic oil which can pass through the valve at the slip control mode is the same as at the normal brake mode.
Therefore, in such a conventional normal-open solenoid valve, an amount of the hydraulic pressure to be supplied cannot be precisely controlled, making it difficult to accurately perform a slip control. Furthermore, a water hammering phenomenon occurs by pulsations of fluid due to the plunger's movement to open and close positions, thereby generating noises and deteriorating the durability of parts.
To solve the above described problems, a normal-open solenoid valve which is designed to reduce an amount of the hydraulic oil to be supplied to the wheel cylinder in proportion to a slip rate, while alleviating pulsations of the fluid during a slip control, has been developed.
As shown in FIG. 1, an example of such a normal-open solenoid valve is disclosed in the U.S. Pat. No. 5,647,644, which comprises a valve seat 23, a magnetic core 21, a plunger 22, and a piston 24, all of which are installed in a valve housing 20. The valve seat 23 is forcedly fitted in a lower portion of the valve housing 20 and provided with a longitudinal through hole. The plunger 22 is mounted passing through the magnetic core 21 such that a lower end thereof is disposed in the vicinity of the top of the valve seat 23. The piston 24 is disposed around the valve seat 23 and biased against the magnetic core 21 by a spring 25.
The normal-open solenoid valve is provided with two orifices, one of which is an invariable orifice formed on an upper end of the valve seat 23 and the other of which is a variable orifice 40 that is defined by a slot 26 formed on the top of the piston 24 when the piston 24 contacts the magnetic core 21 during a slip control.
In addition, a hydraulic channel branch 31 is defined between one side wall of the valve seat 23 and the valve housing 20 so that hydraulic pressure supplied through an inlet 27 can be applied to the piston 24 through the hydraulic channel branch 31, thereby displacing the piston 24 upward while overcoming the biasing force of the spring 25. A stepped portion 23a is formed on another lower side of the valve seat 23 so as to forcedly fit the valve seat 23 into the valve housing 20.
In the normal brake mode, the plunger 22 is urged upward while the piston 24 is biased downward by the spring 25. Therefore, hydraulic oil is supplied through the inlet 27, then directed to an outlet 28 through the invariable orifice 30.
In the slip control mode, the plunger 22 is displaced downward to close the invariable orifice 30. As a result, the hydraulic oil supplied through the inlet 27 is applied to the piston 24 through the hydraulic channel branch 31 so that the top of the piston 24 contacts the magnetic core 21 by being moved upward while overcoming the elastic force of the spring 25. After this, when the plunger 22 is moved upward, the hydraulic pressure generated by the master cylinder is directed to the wheel cylinder through the variable orifice 40 formed by the slot 26 of the piston 24 contacting the magnetic core 21.
When the braking force is released, the hydraulic oil within the wheel cylinder is returned to the master cylinder through the return passage 29 formed passing through the valve housing 20 to communicate the inlet 27 with the outlet 28. After this, the plunger 22 is moved upward so that the solenoid valve is returned to its open state.
However, in the above-described normal-open solenoid valve, since the lower portion of the valve seat is designed to have the stepped portion and the channel branch defining portion, and the channel branch defining portion should be precisely formed, it is difficult to manufacture the valve seat.
In addition, since the special valve housing for receiving the magnetic core, the valve seat, the piston and the fluid passages is required, the entire size of the solenoid valve is increased.